482 wherry: classes of crystals 



some of the attributes of two of them. In table 1 the data in 

 several instances of this relation are presented, the hemihedrism 

 being in every case weak, according to the classification of 

 Professor Goldschmidt.^ 



Diamond, in the majority of text books of mineralogy, is classed 

 as tetrahedral, and Fersmann and Goldschmidt^ concluded from 

 a review of extensive data that it actually possesses weak hemi- 

 hedrism of this type. Attempts to develop piezo- and pyro- 

 electricity have, however, been unsuccessful, ^ which points to 

 holohedrism. In the presence of such conflicting evidence, the 

 assignment of a substance to one class or another has in the 

 past been a matter of opinion on the part of the individual 

 scientist, depending on the relative weights assigned to different 

 features. 



Wlien the study of crystal structure by X-rays became, es- 

 pecially through the brilliant work of the Braggs,^ capable of 

 demonstrating the exact positions of the atoms in crystal space- 

 lattices, there seemed reason to hope that it would be possible to 

 decide such questions, and classify each substance accurately, 

 definitely, and finally. But when the structure of diamond was 

 worked out, it proved, as a whole, to be holohedral, while the 

 structure-units (unit cells of the space-lattice), are tetrahedral 

 in symmetry, leaving the decision as to which class it shall be 

 assigned still in doubt. 



If it be admitted that a substance can belong to two classes 

 at the same time, this difficulty vanishes. There being then no 

 longer need for any evaluation of the relative importance of dif- 

 ferent features in determining the classification of the crystal, 

 the significance of all the physical properties may be considered 

 on an equal footing. Habit, in so far as it represents a condi- 

 tion of equilibrium, should be connected with the structure as a 

 whole; in diamond it should be holohedral, which is actually 

 observed to be the case in the majority of crystals of this sub- 



' GoLDSCHMiDT and NicoL. Neues Jahrb. Min. Geol. 19042; 109; Nies and 

 GoLDSCHMiDT. Op. cit. 1908^:99. 

 2 Der Diament, 1911. 



^ Van der Veen. Zeitschr. Kryst. Min. 51: 545. 1909. 

 * X-rays and Crystal Structure, 1915. 



